This invention relates to miniature portable FM receivers, for example, for one-way paging systems and, more particularly, to circuits and methods for switching such receivers between a synthesized or phase-locked loop (PLL) mode and an automatic frequency control (AFC) mode of controlling the frequency of the local oscillator.
Commonly-assigned application No. 213,719 filed Jun. 30, 1988, now U.S. Pat. No. 4,885,802 entitled "Wristwatch Receiver Architecture," which is now U.S. Pat. No. 4,885,802 issued Dec. 5, 1989 is incorporated herein by reference. That application discloses a miniature portable FM receiver arranged for one-way paging applications. In such a receiver, the local oscillator may be operated in either a PLL synthesized mode or in an AFC mode. In the synthesized mode, a constant reference voltage is provided to one of the two control inputs of a local oscillator (LO). In the AFC mode, the synthesizer is disabled, and an AFC voltage is provided instead to the LO.
Commonly-assigned application Ser. No. 213,719 filed Jun. 30, 1988, which has been issued as U.S. Pat. No. 4,885,802, discloses the concept of shutting off the frequency synthesizer when the FM receiver is receiving data to conserve power and prevent noise from the synthesizer from interfering with or degrading the data. When the synthesizer is disabled, the LO must be switched from synthesizer to AFC mode.
In a system where the circuit shown in the previously referenced patent application is implemented using low voltage circuitry which operates in a differential mode, the design of a switch circuit for LO mode switching presents several technical challenges. Known analog switches such as the National Semiconductor LF13508 analog multiplexer require a relative high supply voltage, and such circuits only have single input terminals. Other known differential analog multiplexers, for example, the National Semiconductor LF13509, provide differential inputs and outputs, whereas the LO signal which is being switched in the present application requires that the AFC switch have a single ended output.
Another requirement is that AFC switch circuitry have low offset. Because of very high gain in the LO, an offset error in the control voltage may force the LO so far off the desired frequency in AFC mode that the feedback system derails. In that event, the receiver would be useless until the synthesizer restarts.
Yet another requirement is high isolation from input to output; that is, when one input is selected, there must be high isolation from the unselected input to the output.